Plasma processing of semiconductor wafers in the manufacture of microelectronic integrated circuits is used in dielectric etching, metal etching, chemical vapor deposition and other processes. Such plasma processes require precise control of process parameters, such as the amount of plasma power delivered to the plasma, for example. This parameter is affected by a number of variables, including the ability of the impedance matching device between the plasma source power generator and the reactor's RF power applicator to provide an impedance match over a widely varying plasma load impedance. The wide range of plasma load impedance is attributable to changing conditions within the reactor chamber. As described in U.S. Pat. No. 6,528,751 referenced above, this problem is addressed by a fixed impedance match device, such as a coaxial tuning stub or a strip line circuit, that couples source power to the ceiling electrode and has a wide match space. As described in the referenced patent, the reactance of the electrode is selected so that the electrode and plasma resonate at a plasma electrode resonant frequency. Further, the resonant frequency of the fixed match device, the electrode-plasma resonance and the source power frequency are all nearly equal and lie in the VHF range. One advantage is that the fixed match device has a very wide match space, so that the system is less sensitive to variations in plasma load impedance (so that such variations do not greatly affect the amount of source power delivered to the plasma). Even greater imperviousness to variations in plasma load impedance is obtained by providing a slight deviation between these three frequencies, as described in the above-referenced patent.
A problem limiting the process window of such a reactor is that the electrode-plasma resonance frequency varies widely with chamber pressure. What is desired is a plasma reactor that can perform a process, such as a reactive ion etch process, over a wide process window, including a wide range of chamber pressures from about 5 mT to about 1000 mT. The problem is that such a wide variation in pressure creates changes in the plasma-electrode resonant frequency that cause an impedance mismatch and consequent loss of control over delivered plasma source power. As described below in the detailed description, the fixed impedance match (i.e., the coaxial tuning stub) has a wide impedance match space provided that the plasma-electrode resonant frequency does not deviate too far from the source power frequency. In general, the fixed impedance match need only hold the VSWR at the source power generator within 3:1 to provide an adequate match. What we have found is that as chamber pressure is varied from 5 mT to 1000 mT, the VSWR exceeds 3:1 over a portion of this range. Thus, the chamber pressure must be confined within a much smaller range.
We have found that the variation in plasma-electrode resonant frequency with chamber pressure seems to be unavoidable: The resonant frequency is a function of the plasma impedance which in turn depends upon the electron-neutral collision frequency. The electron-neutral collision frequency is a direct function of chamber pressure. Thus, a variation in plasma-electrode resonant frequency with chamber pressure would appear to be unavoidable, so that realizing a wide pressure window (e.g., 5–1000 mT) has not seemed an attainable goal.